This invention relates to a transfer arrangement composed of electrically conductive members such as a transfer and transport belt and a transfer roller, and relates to an image forming apparatus using the transfer arrangement.
In an image forming apparatus such as a color electrophotographic printer (a tandem type printer) in which a recording medium is transported along a single path, a transfer voltage is applied by a transfer power source to a photosensitive drum and a shaft of a transfer roller. A transfer and transport belt (hereinafter, referred to as a transfer/transport belt) and a recording medium are nipped by the transfer roller and the photosensitive drum. Due to the transfer voltage, a toner is transferred from the photosensitive drum to the recording medium.
As the toner moves from the photosensitive drum to the recording medium, and an electric charge also moves from a part of the surface of the photosensitive drum (where the toner does not exist) to the recording medium, the current flows between the photosensitive drum and the recording medium. This current is referred to as a transfer current. There is a close relationship between the transfer current and printing quality. In the color electrophotographic printer, transfer units of black, yellow, magenta and cyan respectively have transfer power units, and the transfer voltages applied by the transfer power units are individually controlled so as to generate the optimum transfer currents.
A transfer arrangement of each transfer unit is composed of electrically conductive members, i.e., a transfer/transport belt and a transfer roller. The transfer/transport belt contacts the recording medium and the photosensitive drum (i.e., a toner image bearing member). The transfer roller does not directly contact the toner image bearing member, but forms a suitable nip against the toner image bearing member. The transfer roller is made of a conductive shaft and a conductive resilient portion formed on the conductive shaft.
Conventionally, the conductive resilient layer of the transfer roller is made of, for example, insulation material such as silicone, polyurethane, epichlorohydrin, NBR (nitrile-butadiene rubber), EPDM (ethylene-propylene-diene monomer) to which electrolyte (such as salt including an element of group 1 or 2 of the periodic table or ammonium salt), electrically conducive polymer or carbon black is added as conductive material.
Further, the transfer/transport belt is made of, for example, insulation material such as polycarbonate (PC), polyvinylidene fluoride (PVDF), polyimide (PI), polyamideimide (PAI) or ethylene tetrafluoroethylene (ETFE) to which carbon black is added as conductive material.
Conventionally, there is a type of transfer arrangement made of an electrically conductive member having a characteristics that current increases in ohmic way as the applied voltage increases but resistance (i.e., electric resistance) does not change even when the applied voltage changes. There is another type of transfer arrangement made of an electrically conductive member of high resistance having a characteristics that resistance changes as the applied voltage changes (i.e., resistance is controlled by a semiconductive region). In the electrically conductive member of high resistance, the current increases exponentially as the applied voltage increases.
It is important to comprehend the above-described characteristics that the current increases exponentially as the applied voltage increases. This is because whether the transferring is excellently performed or not depends on whether the correct transfer current is generated or not. The more rapid the current changes, the narrower the range of the transfer voltage becomes, and therefore it becomes difficult to adjust a transfer table. Because of these reasons, the above described characteristics is one of the most important parameters of the characteristics of the transfer arrangement.
Moreover, there is another reason why the above characteristics is one of the most important parameters as follows. A predetermined voltage is applied to the photosensitive drum and the shaft of the transfer roller. When the printing is performed on a recording medium (such as a postcard) having a narrow width and high resistance, a voltage applied to a part of the transfer arrangement on which the recording medium does not exist is lower than a voltage applied to a part of the transfer arrangement on which the recording medium exists, and a different therebetween is proportional to resistance of the recording medium. If the current is expressed as an exponential function of the applied voltage, the current tends to be larger in the part of the transfer arrangement on which the recording medium does not exist than in the part of the transfer arrangement on which the recording medium exist. Thus, in the total amount of the measurable current, an amount of the current resulting from the movement of the toner is small. Accordingly, the transfer efficiency is low, even when the total amount of the transfer current is large. Moreover, when the transfer voltage increases, the amount of the current flowing into the non-print region of the transfer arrangement also increases, and may cause an electric shock (i.e., a transfer shock) on the photosensitive drum. From this viewpoint, the above characteristics is one of the most important parameters of the characteristics of the transfer arrangement.
A voltage dependence ΔR of the resistance (i.e., the dependence of the resistance on the voltage) is defined for relatively comparing the degrees of the changes of the resistances with respect to the applied voltages. Comparison voltages respectively higher and lower than a voltage that causes a predetermined current (for example, 10 μA) to flow are expressed as V1 and V2 (=2×V1). The resistance at the comparison voltage V1 is referred to as R1, and the resistance at the comparison voltage V2 is referred to as R2. The voltage dependence ΔR of the resistance (hereinafter, simply referred to as voltage dependence ΔR) is expressed as follows:ΔR=(R(V1)−R(V2))/R(V1)
In the case of the electrically conductive member whose resistance is controlled by a semiconductive region, the lower the voltage dependence ΔR is, the higher the transfer efficiency becomes.
The resistances R (V1) and R (V2) of the transfer roller are measured in the same direction as the transferring of the toner in the transfer unit, on condition that the transfer roller contacts a drum-shaped metal and rotates together with the drum-shaped metal at temperature of 20 degrees centigrade and at humidity of 50%. The resistances R (V1) and R (V2) of the transfer/transport belt are measured in the same direction as the transferring of the toner in the transfer unit, on condition that the transfer/transport belt is nipped by two rotating drum-shaped metals at temperature of 20 degrees centigrade and at humidity of 50%.
Further, the voltage dependence ΔR of the transfer arrangement (composed of a plurality of electrically conductive members) is obtained by measuring the relationship between the applied voltage and the generated current of each electrically conductive member, and by combining the results of the electrically conductive members. The resistances R (V1) and R (V2) (or the comparison voltages V1 and V2) can be suitably chosen from higher and lower values respectively lower and higher than the resistance (or the applied voltage) that causes a target current to flow.
The reason of choosing the current of 10 μA is that the current corresponds to (i.e., substantially equals to) the transfer current in the printer. The charging amount of the toner and the charging amount of the elements of the respective electrophotographic processes vary with the type of the printer, and therefore the optimum current varies with the type of the printer. In such a case, it is preferable to determine the voltage dependence ΔR based on the lower and higher resistances (or voltages) respectively lower and higher than the resistance (or voltage) that causes the optimum transfer current to flow.
The conventional transfer arrangement composed of an electrically conductive member whose resistance is controlled by the semiconductive region generally has a high voltage dependence ΔR. For example, in the case of a conventional transfer roller having a conductive resilient portion made of EPDM (ethylene propylene diene monomer) to which carbon black is added, the voltage dependence ΔR is 0.75. In the case of a conventional transfer/transport belt made of polyamide to which carbon black is added, the voltage dependence ΔR is 0.86. The voltage dependence ΔR of both of the transfer roller and the transfer/transport belt are high. The voltage dependence ΔR of the conventional transfer arrangement (combining the transfer roller with the voltage dependence ΔR of 0.75 and the transfer/transport belt with the voltage dependence ΔR of 0.86) is 0.78. Thus, the voltage dependence ΔR of the conventional transfer arrangement is high.
Conventionally, there is a type of the transfer roller having a conductive resilient portion to which electrolyte or electrically conducive polymer is added for lowering the voltage dependence ΔR. As the transfer arrangement includes the transfer roller whose voltage dependence ΔR is lowered by adding electrolyte or electrically conducive polymer, it is possible to lower the voltage dependence ΔR of the transfer arrangement whose resistance is controlled by the semiconductive region. In the case where the electrically conductive member is made of a conductive material having ohmic character, the voltage dependence ΔR is 0.
The example of the above described conventional transfer arrangement is disclosed in Japanese Laid-Open Patent Publication No. 2002-14543.
However, in the conventional transfer arrangement whose voltage dependence ΔR is 0 (i.e., the transfer arrangement made of a conductive material having ohmic character), very high transfer voltage is needed to generate the optimum transfer current. As a result, the load on the transfer power source may increases, and the lifetime of the transfer arrangement may be shortened.
Moreover, in the conventional transfer arrangement whose voltage dependence ΔR is high (i.e., whose resistance is controlled by the semiconductive region), it is possible to lower the transfer voltage, but the leakage of the transfer current may occur in the vicinity of the end portion in the width direction of the recording medium, with the result that the transfer efficiency of the toner may decrease. In particular, if the printing is performed on a thick paper having a narrow width, a back side of a postcard, or an end portion of a special media (for example, a film or an OHP sheet), a transferred image may become blurred, and therefore the printing quality may be degraded.
Additionally, it becomes possible to obtain a sufficient printing quality on the above described recording media, by using the conventional transfer roller having the conductive resilient portion to which electrolyte or electrically conducive polymer is added. However, in order to add electrolyte or electrically conducive polymer to the conductive resilient portion of the transfer roller, the solubility to an insulation material is required. Therefore, the choice of the insulation material, the electrolyte and the electrically conducive polymer are limited. Thus, compared with the carbon black, the electrolyte and the electrically conducive polymer may become expensive, and therefore the cost of the transfer roller increases.